Distribution & Dominant Species in Pacific Northwest Forest Systems

[The following summary is from Vose et al., 2012, pp. iv-v]
“A gradual increase in temperature will alter the growing environment of many tree species throughout the United States, reducing the growth of some species (especially in dry forests) and increasing the growth of others (especially in high-elevation forests). Mortality may increase in older forests stressed by low soil moisture, and regeneration may decrease for species affected by low soil moisture and competition with other species during the seedling stage. Most models project that species habitat will move upward in elevation and northward in latitude and will be reduced in current habitats at lower elevations and lower latitudes. New climatic conditions may “move” faster in some locations than tree species can disperse, creating uncertainty about the future vegetation composition of these new habitats.

The high genetic diversity of most tree species confers tolerance of a broad range of environmental conditions, including temperature variation. Therefore, in many species, tree growth and regeneration may be affected more by extreme weather events and climatic conditions than by gradual changes in temperature or precipitation. Longer dry seasons and multiyear droughts will often become triggers for multiple stressors and disturbances (e.g., fire, insects, invasive species, and combinations thereof). These pulses of biophysical disturbance will change the structure and function of ecosystems across millions of hectares over a short period of time, focusing pressure on the regeneration stage of forest ecosystems. Increased atmospheric carbon dioxide (CO2) and nitrogen deposition will potentially alter physiological function and productivity of forest ecosystems, with considerable variation in response among species and regions.”

For example, climate suitability for Douglas-fir is projected to shrink 32 percent of its current range in the State of Washington, and up to 85 percent of the range of some pine species may be outside the current climatically suitable range (Littell et al. 2010, Rehfeldt 2006).

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